Tux3: the other next-generation filesystem
Posted Dec 16, 2008 1:42 UTC (Tue) by daniel
In reply to: Tux3: the other next-generation filesystem
Parent article: Tux3: the other next-generation filesystem
I've only lived with maybe a few dozen disks in my life, but I've still corruption like that too -- in this case, it turned out that the disk was fine, but one of the connections on the RAID card was bad, and was silently flipping single bits on reads that went to that disk (so it was nondeterministic, depending on which mirror got hit on any given cache fill, and quietly persisted even after the usual fix of replacing the disk).
Luckily the box happened to be hosting a modern DVCS server (the first, in fact), which was doing its own strong validation on everything it read from the disk, and started complaining very loudly. No saying how much stuff on this (busy, multi-user, shared) machine would have gotten corrupted before someone noticed otherwise, though... and backups are no help, either.
Our ddnap-style checksumming at replication time would have caught that corruption promptly.
if there comes a day when there are two great filesystems and one is a little slower but has checksumming, I'm choosing the checksumming one. Saving milliseconds (of computer time) is not worth losing years (of work).
It is not milliseconds, it is a significant fraction of your CPU, no matter how powerful. But yes, if you want extra checking is important to you, should be able to have it. Whether block checksums belong in the filesystem rather than volume manager is another question. There may be a powerful efficiency argument that checksumming has to be done by the filesystem, not the volume manager. If so, I would like to see it.
Anyway, when the time comes that block checksumming rises to the top of the list of things to do, we will make sure Tux3 has something respectable, one way or another. Note that checksumming at replication time already gets nearly all the benefit at a very modest CPU cost.
If you want to rank the relative importance of features, replication way beats checksumming. It takes you instantly from having no backup or really awful backup, to having great backup with error detection. So getting to that state with minimal distractions seems like an awfully good idea.
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